1. Field of the Invention
The present invention relates to a control circuit for a circuit interrupter and, more specifically, to a control circuit for selectively applying energy to operate a circuit interrupter in response to, and a selected time after the onset of, overcurrent in an alternating-current line in which the circuit interrupter is located. As used herein, "overcurrent" means current exceeding a threshold value. The threshold value may be determined, for example, by the maximum current deemed acceptable in view of the characteristics of the circuit interrupter and of the equipment in the line being protected thereby.
2. Description of the Prior Art
Some prior art protective devices, such as fuses, circuit breakers, circuit switchers, and reclosers, may operate according to an inverse time-current relationship. Specifically, the higher the level of overcurrent in a line, the more quickly the protective device operates. Conversely, the lower the level of overcurrent in the line, the more slowly the protective device operates. Other prior art protective devices respond to overcurrent in the line "instantaneously", that is, with minimum or no time delay after the inception of overcurrent in the line. However, many prior art protective devices do not operate both instantaneously as well as according to an inverse time-current relationship in such a way as to coordinate with other protective devices and damage curves of equipment being protected.
It is therefore an object of the present invention to provide a control circuit that operates according to both an inverse time-current relationship and an instantaneous relationship, the former relationship resulting from "moderate" (as hereinafter defined) overcurrent in a line and the latter relationship resulting from "high" (as hereinafter defined) overcurrent in the line.
Further, it is desirable that the control circuit be of a reasonable size and be as simple as possible so that it is adaptable for various applications and is relatively reliable and inexpensive to manufacture.
Yet another object of the present invention is to provide a versatile control circuit that may be used to control the operation of or otherwise affect a wide variety of devices other than circuit interrupters.
Still another object to the present invention is to provide a control circuit for use with a circuit interrupter or other device and having both a time-current relationship and an instantaneous relationship that are selectively variable to match the damage curves of equipment connected to a line and that can be coordinated with other protective devices in the line.